1. Field of the Invention
The present invention relates to a sheet output apparatus which binds plural sheets of paper to generate a set of sheets and outputs the generated set of sheets.
2. Description of the Related Art
Generally, an example of relatively frequent office work includes production of a booklet which is performed by binding plural sheets in which the images are formed with an image forming apparatus such as a printer and a copying machine. Conventionally, in many cases, the sheet output apparatus is used as the apparatus which easily performs the binding work. The sheet output apparatus has a mechanism (hereinafter referred to as stapler) which binds the plural sheets by a binding needle (hereinafter referred to as staple) to generate the set of sheets, and the sheet output apparatus outputs the set of sheets generated with the stapler.
Some sheet output apparatuses have a first tray on which the plural sheets are temporarily loaded until the plural sheets are stapled and a second tray in which the stapled set of the plural sheets is finally stored. The generated set of sheets is nipped between a pair of rollers opposite to each other to be output from the first tray toward the second tray. In such sheet output apparatuses, when a size of the sheet or the number of sheets is large, there is a possibility that so-called stack failure is generated in which positions of the output set of sheets are varied on the second tray. Therefore, a technique in which, when the size of the sheet or the number of sheets is large, the set of sheets is output at low speed from the first tray toward the second tray is proposed in order to prevent the stack failure from occurring (for example, see Japanese Patent Application Publication No. 8-5585, pages 2 to 4 and FIG. 5).
Some image forming apparatuses have a function of forming the images on both sides of the sheet. However, when the set of sheets is generated by stapling the plural both-sides sheets in which the images are formed on both sides by the image forming apparatus with stapler to output the set of sheets, there is often generated the problem that the uppermost sheet and the lowermost sheet which are in contact with the pair of rollers for nipping the set of sheets are output while slipped from the set of sheets and the stapled portion is broken. As with the case of the stack failure, the problem can be prevented by outputting the set of sheets at low speed. However, in order to sufficiently prevent the generation of the problem, it is necessary to output the set of sheets at too slow speed, which generates another problem that productivity is remarkably decreased.